Coded apertures have been used in astronomical imaging for a number of years. In these applications, imaging is performed in very short-wave spectral ranges, such as X-rays, where diffraction is negligible.
When coded-aperture imaging is attempted in the infrared spectral range, the effect of diffraction is significant and may pose a problem. Such imaging in the presence of diffraction is of practical interest. (Tim Clark and Esko Jaska, DARPA Interest in Diffractive Sensors, Proc. of SPIE Vol. 6714, 671403, (2007))
Diffraction causes a blur in image on the focal plane array (FPA). Slinger et al. used an approach to deblurring the image in the process of coded-aperture decoding, with Fourier deconvolution and noise reduction by Tikhonov regularization on multiple captured FPA frames (C. W. Slinger, M. Eismann, N. Gordon, K. Lewis, G. McDonald, M. McNie, D. Payne, K. Ridley, M. Strens, G. De Villiers, R. Wilson, An investigation of the potential for the use of a high resolution adaptive coded aperture system in the mid-wave infrared,” Proc SPIE 6714-07 (2007) and C. W. Slinger, G. D. De Villiers, D. A. Payne, Processing method for coded aperture imaging, WO 2007/091049).
A different approach disclosed in a PCT publication by Slinger (C. W. Slinger, Imaging System, WO 2007/091051) uses a coded diffractive mask designed such that its diffraction pattern at the waveband of interest is a well-conditioned coded intensity pattern having a strong autocorrelation function with low sidelobes. Radiation reaching the detector array is diffracted by the diffractive mask but in a defined way, and it is the diffraction pattern of the mask which provides the coding. The scene image can then be reconstructed using the same techniques as for conventional coded aperture imaging but using the diffraction pattern of the mask as the aperture function. The diffractive coded aperture mask in Slinger's invention acts in effect as a hologram that is reconstructed by a plane wavefront from a distant target to produce a well-resolved coded pattern on the FPA. The well-resolved pattern is a traditional coded aperture pattern. The coded pattern on the FPA is then processed the same way as X-ray and other “diffraction-free” coded-aperture images.
It is known from holography that an aberration-free reconstructed image can only be produced when the reconstructing wavefront is of exactly the same nature as the reference wavefront that was used during the hologram recording. For example, if a plane wavefront is used as the reference for recording, a plane wavefront of exactly the same orientation during reconstruction is required to produce an aberration free image. If the reconstructing wavefront arrives at the hologram at a different angle, the image will be aberrated. This limits the field of view of the invention disclosed in WO 2007/091051, where different points of the field will produce different “reconstructing wavefronts” of the coded-aperture mask “hologram.” Only for one look angle can the “hologram” mask be designed to produce an aberration-free array image on the FPA. At other look angles, aberrations may be significant, increasing with the deviation of the look angle from the design value, as well as with the aperture size of the “hologram” array mask.
Slinger's PCT publications WO/2007/091049, WO2007/091047, WO2006/125975, and WO/2007/091051 disclose an imaging system where the coded-aperture mask acts as a diffractive optical element, or hologram, so that radiation reaching the FPA, diffracted by the mask, produces a well-conditioned intensity pattern on the FPA, with strong autocorrelation and low sidelobes. As between 1) feature size of the diffracted pattern on the FPA and 2) FPA pixel pitch, the larger (coarser) of these two determines the angular resolution.
Slinger's invention is prone to aberrations at look angles substantially different from the look angle for which the diffractive mask is designed. For every different look angle, Slinger's imaging system in principle requires changing the diffractive pattern on the mask. Slinger's inventions produce intensity images; sensing the phase of an arriving wavefront is not provided.
Slinger's diffractive masks, due to their binary nature (transparent versus opaque, bit depth of 1), produce higher diffraction orders with noise-like stationary image artifacts. The use of low-noise sensors does not reduce this detrimental effect. This problem is mitigated by capturing and statistical treatment of multiple images of the same scene, with different, dynamically changing, adaptive mask patterns. This mitigation, however, requires complex adaptive masks (e.g., using micro-electromechanical, MEMS, devices), stationary objects that do not move between frames, and a stable platform on which the imaging system is installed.
Slinger's imaging, as described in these patent publications, also does not provide color information.
It is an object of this invention to provide a coded aperture imaging system with high imaging resolution in the visible and infrared spectral ranges, where there may be significant diffraction.
Another object of this invention is to achieve incident wavefront sensing, including amplitude and phase.
Another object of this invention is to remove aberrations caused by windows or optics present in the optical path preceding the aperture of the system.
Another object of this invention is to remove aberrations caused by atmospheric turbulence in the optical path preceding the aperture of the system.
Another object of this invention is to provide synthetic-aperture imaging with multiple sensing apertures jointly forming a high resolution image of a scene.
Another object of this invention is to provide a coded-aperture imaging system operable in the ultraviolet, visible, infrared, and longer-wavelength spectral ranges with a simplified coded aperture mask.
Another object of this invention is aberration-free imaging, over extremely wide fields of view, with a single coded-aperture mask.
Another object of this invention is to achieve diffractive coded-aperture imaging free of noise-like image artifacts.
Another object of this invention is to provide color, spectral, and hyperspectral sensing and imaging.
Another object of this invention is to provide three-dimensional imaging of objects.
Another object of this invention is to provide polarization-sensitive imaging and wavefront sensing.
Finally, another object of this invention is to provide coded-aperture imaging with automated detection of change.